According to JP-A-2004-12299 (U.S. Pat. No. 6,911,798 B2), an electric motor operates a shift range switching device for an automatic transmission of a vehicle. In this structure, an electric motor is supplied with electricity to perform a tapping control, when a reference position of a rotor or a shift position is unknown in a starting condition of the vehicle. During the tapping control, the rotor is rotated to a limit position on one side in a movable range of the shift range switching device, for example, so that the rotor is rotated until the rotor reaches at the limit position. The limit position is on the side of the parking position, for example. The position, in which the rotor stops, is defined as one of the reference position in the rotation control of the rotor and a reference position of the shift switching control, during the tapping control.
When the tapping control is performed, the following problems may arise. First, when a movable member collides against a fixed member in the tapping control, a mechanical load is applied to both the movable member and the fixed member. Alternatively, when the movable member is stopped while an electric motor is supplied with electricity, load torque is applied to components in both a transmission system for rotating the movable member and a hooking portion, in which the movable member hooks to the fixed member, due to torque applied by the electric motor. Accordingly, as the number of the tapping control increases, mechanical damage may occur in the components of the transmission system and in the hooking portion, in which the movable member hooks to the fixed member. As a result, the components of the transmission system and the hooking portion may be gradually deformed and broken.
Second, output voltage and a capacity of a power source such as a battery may vary in dependence upon the environment such as atmospheric temperature and an operating condition. In this case, electricity supplied to the electric motor from the power source may vary, and output torque of the electric motor may change. Specifically, the output torque of the motor may increase in dependence upon the environment and the operating condition. In this situation, mechanical load may increase when the movable member collides against the fixed member. Alternatively, large mechanical torque may be applied to the components in the transmission system and the hooking portion, because of supplying electricity to the electric motor even when the movable member stops. Accordingly, as the number of the tapping control increases, the number of applying large load torque increases, and as a result, mechanical damage may occur in the components of the transmission system and the hooking portion.
In addition, the components are not necessarily perfect rigid bodies. That is, the components are macroscopically spring elements, and the components may cause deflection when being applied with force. When output torque of the motor varies, an amount of deflection arising in the components varies. As a result, the reference position, which is learned during the tapping control of the movable member, cannot be stable. Thus, a positioning control of the movable member cannot be steadily performed.
Third, as the rotation speed of the electric motor increases, the output torque of the electric motor decreases. By contrast, as the rotation speed of the electric motor decreases, the output torque of the electric motor increases. Accordingly, when the movable member stops while the electric motor is supplied with electricity, the electric motor generates the maximum torque. Consequently, large mechanical torque is applied to the components of the transmission system and the hooking portion between the movable member and the fixed member. Accordingly, as the number of the tapping control increases, the number of applying large load torque increases. Consequently, mechanical damage may occur in the components of the transmission system and the hooking portion.